A high-precision and high-resolution laser-scanning optical system is used in office automation equipment including a laser printer and a laser facsimile, and in such apparatuses as a laser drawing apparatus and a laser inspection apparatus. Conventionally, this optical system is embodied by a rotating polygonal mirror and a combination of a plurality of f-.theta. lenses.
In the above method employing a polygonal mirror, efforts to lower cost have met with difficulty because of the high precision required to fabricate a rotating polygonal mirror and because of a large number of lens groups required, including f-.theta. lenses that serve, at the same time, as inclination correction optical system.
On the other hand, a hologram scanning apparatus employing a hologram can be mass produced. As an example of such a hologram scanning apparatus, the present applicant has filed an application for a hologram scanning apparatus for performing a scanning with a straight beam having a high resolution and having sufficiently corrected aberration (the Japanese Laid-Open Patent Application 63-072633 and the Japanese Laid-Open Patent Application 61-060846). This light beam scanning apparatus achieves, as a scanning optical system for a laser printer, excellent specifications characterized by a high precision, ensuring a stable print quality. However, there is now a demand for a laser-scanning optical system having even higher resolution, on the order of 400-600 dpi or even 1000 dpi. Also, further cost reduction is desired.
In order to embody a hologram scanner having such an extremely high resolution at a low price, the following objectives need be resolved:
1 scanning beam radius should be as thin as 60 .mu.m (equivalent to 400 dpi), for example, and as uniform as possible; and
2 a scanning should be carried out at the same velocity as that of the rotation of a rotatable hologram, which rotation is at constant angular acceleration.
Since a wavelength of a semiconductor laser used therein as a scanning light source can vary according to ambient temperature and since several longitudinal modes can be produced,
3 displacement in a scanning direction of a scanning beam should be compensated for; and
4 displacement in a cross scanning direction of a scanning beam should be compensated for.
Since a scanning beam displacement is attributable to a warping of a base used in a rotatable hologram and the warping takes place as a result of using a floating glass, which is of low cost and needs no polishing, or a plastic base (PMMA, for example) enabling injection molding,
5 a scanning beam displacement due to the plastic base being moved from its ideal position should be compensated for.
The present applicant had proposed a method of achieving the above tasks in the Japanese Laid-Open Patent Application 58-119098. The device used in the method comprises, as shown in FIG. 14, a rotatable hologram 10 and a fixed hologram plate 20 disposed between the rotatable hologram 10 and an image formation surface 4. The hologram 10 is a rapidly rotating rotatable hologram in which a plurality of hologram plates are disposed. Further, 5 is a reconstructing beam, 6 is a diffracted wave outgoing from the hologram plate 10, and 7 is a diffracted wave outgoing from the fixed hologram plate 20. The reconstructing beam from a semiconductor not shown in the figure is diagonally incident on the rotating rotatable hologram 10, whose rotation enables the scanning by the diffracted wave 6. The diffracted wave 6 is incident on the fixed hologram plate 20, and the diffracted wave 7, which is a wave diffracted therefrom, scans the image formation surface 4.
In the above configuration, displacement of a scanning beam position due to a wavelength variation of the semiconductor laser is compensated for, and a velocity of the scanning beam is maintained constant by a rotation of constant angular acceleration of the rotatable hologram 10, so that a straight-line scanning by a scanning beam is achieved. Further, displacement of a scanning beam position both in the scanning direction and the cross scanning direction, which displacement is due to a wavelength variation of the semiconductor laser, is corrected by having the fixed hologram plate 20 bend the scanning beam in a direction counter to a scanning direction of the rotatable hologram 10.
As an improved method of compensating for displacement of the scanning beam position in the cross direction due to a wavelength variation of the semiconductor laser, the present applicant filed an application for the Japanese Laid-Open Patent Application 60-168830, in which it is proposed that a fixed hologram plate be spatially placed before the rotatable hologram.
The present applicant also made a proposition in the Japanese Laid-Open Patent Application 2-179437 (the domestic declaration of priority on the Japanese Laid-Open Patent Application 1-240720), in which is proposed a construction capable, by employing at least two holograms, of maintaining uniform optical path lengths from an incident wave to an image formation surface, and of preventing degradation of wavefront characteristics on the image formation surface, which degradation is caused by a wavelength variation of the reconstructing light source. Since the Japanese Laid-Open Patent Application 2-179437 relates to an optical system where at least two holograms, as mentioned above, are fixed, and therefore only one image formation point is provided, an application of the same device to the scanning optical system now being discussed entails some difficulty in that moment-by-moment optical path length changes, which take place as the beam scanning proceeds, inevitably cause the optical path length to be longer at the scanning end than at the scanning center. Accordingly, the aforementioned conventional technology has not resolved all of the objectives from 1 through 5 described earlier.